In case of newly realizing a low frequency radar in Japan, it is required to use a low radio frequency band that can avoid the frequency bands used by a large number of existing radio stations. This is the reason why the radar transmission signal must unavoidably be discrete bands. Proposed in JP2002-82162A entitled “Pulse Compressed Radar System” is a technique for effectively improving display accuracy of targets by allocating only effective frequency bands in the radar transmission signal and eliminating invalid frequency bands that do not provide clear displays because frequency bands to provide clear displays varies depending upon targets in case of using discrete frequency bands.
When generating a transmission signal waveform in, for example, a band B in a general radar system, it is normal to adopt a chirp signal in which the frequency varies continuously within a transmission time of τ=1/B. Although the chirp signal is able to simultaneously achieve both wide bandwidth and large output, there is a possibility to receive a receiving wave (or a reflection wave) from near targets when using a narrower band and a longer transmission time because the reflection wave is received after termination of the transmission signal as shown in FIG. 7. FIG. 7 illustrates the relationship between the transmission time of the transmission signal and the receiving signal in the radar. FIG. 7A is an example of transmitting the transmission signal of a band B over a relatively short time τ(=1/B), while FIG. 7B illustrates an example of transmitting the transmission signal of a band B over a relatively long time τa (>1/B).
As apparent from FIG. 7A, if the time τ(=1/B) is relatively short, the transmission operation is terminated before reflection waves from near targets arrive, thereby enabling to receive reflection waves from near targets as well as those from far targets without any trouble. On the other hand, if the transmission time τa (>1/B) is relatively long as shown in FIG. 7B, the transmission operation is not terminated yet when reflection waves from near targets arrive, thereby disabling to detect near targets at locations corresponding to the shaded zone in FIG. 7B.
In contrast to the foregoing, when generating a transmission signal waveform of discrete bands b1, b2, . . . , bn within a band B and continuously sweeping the frequency in the same manner as the chirp signal, there requires a longer transmission time τ as shown in FIG. 8B:τ=τ1(=1/b1)+τ2(=1/b2)+ . . . +τn(=1/bn)This means that non-receivable near zones increase. Accordingly, in order to detect near targets, it is necessary to make the transmission time as short as possible in case of generating the discrete bands transmission signal. FIG. 8 is an illustration of transmitting the discrete bands transmission signal by continuously sweeping the frequency. FIG. 8A is an example of continuously varying frequencies in the band B. On the other hand, FIG. 8B is an example of continuously sweeping discrete signals within the band B over the respective required transmission times.
In other words, in case of transmitting the transmission signal of continuous band B as shown in FIG. 8A, it requires the shortest transmission time τ(=1/B) as described hereinabove with reference to FIG. 7. And the signal is transmitted by continuously varying frequencies as a chirp signal over a time τa (>1/B) that is larger than the minimum time τ. On the other hand, when transmitting the discrete bands transmission signal within the band B continuously for each discrete band in the same manner as the chirp signal as shown in FIG. 8B, it is required to transmit the respective discrete bands over the transmission time longer than the total of their shortest transmission times τi=1/bi (where, i=1˜5). As a result, it takes undesirably longer transmission time than preferable.